Drain pineapple, reserving juice; set pineapple aside. Place juice in a saucepan/ add butter, sugar and lemon juice. Bring to a boil. Combine cornstarch and cold water until smooth; add to the saucepan, stirring constantly. Return to a boil; cook and stir for 2 minutes. Chill. Stir in mayonnaise. In a large bowl, combine pineapple, apples, grapes, poppy seeds and cooked dressing. Fold in pecans just before serving.